I spend a lot of time on public transportation. I take the bus. I take Muni's little trains. I take BART.

I take it all, and that appears to be getting more unusual by the day. Yes, there are a lot of people who say they take "public transportation," but they really only ride on Muni trains, or on BART. They do not and will not take the bus.

I watch people's faces when they see me waiting at a bus stop. Many of them, especially drivers, look at me like I'm doing something vaguely unsavory - like I'm drinking out of a paper bag or flashing "designer" watches for sale.

To many people, taking the bus is on the same level as these activities. It's interesting to ask people why they won't take it. Usually their objections seem practical, at least on the surface: The bus is "slow" or it's "always late."

All of this is true and rooted in fact. I asked Max Hirsh, a transportation researcher at the Swiss Federal Institute of Technology Zurich, and he explained that bus routes tend to be designed for people who have "a smaller radius of operations," and for "people who have more time than money."

In other words, poor people.

That's the real reason why many residents avoid the bus, isn't it? Not that it's slow or unreliable - anyone who's taken Muni or BART trains knows that it's not just bus drivers who don't show up on time. Plus, if you need a train going in a specific direction, the wait can be endless. All of the systems are subject to breakdowns, to overcrowding, to all sorts of unpleasantness that can keep you from getting quickly to your destination.

The real reason why people give me that look when they see me waiting at the bus stop is what happens after I get on the bus. It's the people who are taking up two or three seats with thousands of plastic bags from low-budget food markets. It's the people who smell like they've been living on the street. It's the people who have loud cell phone conversations about their court cases or their overdue child support.

Now, I understand that such passengers can take up a lot of room, literally and metaphorically. They have different problems than you and I. But what's the big deal about that? That doesn't strike me as a good enough reason to take a train and never a bus.

San Francisco's fabulous public transportation system is a myth in certain ways, because the trains cover only a limited area, and there are plenty of neighborhoods that are served only by bus. At some point, isn't the bus more convenient?

When I ask people that, that's when I hear about what people really think about the bus. It turns out that what they really think is that the bus is "too rowdy." Some mention the "danger" of "people fighting" or "doing graffiti." Some mention the "danger" of their smartphones being snatched. The train weeds out the kinds of people who do these things - at least in their perception. And if the train can't get you to where you want to go, maybe that's when it's time to start thinking about a car.

Most people won't talk about the bus beyond this point, because most people who live in San Francisco like to believe that they have progressive politics. It's quite difficult for them to admit the awful truth: They simply don't like to be in the same space as poor people.

I watch people's faces when they see me waiting at a bus stop. Many look at me like I'm doing something vaguely unsavory.